The present invention is directed to a method and apparatus for epoxidizing an unsaturated oil or an alkyl fatty acid ester, particularly an unsaturated vegetable oil, such as soybean oil, linseed oil, or an ester of tall oil fatty acids. More particularly, the invention is directed to a thin-film method of epoxidizing an oil or an alkyl fatty acid ester by combining the oil or alkyl fatty acid ester with an oxidizing agent, such as hydrogen peroxide, for reaction (epoxidation) with the oil or alkyl fatty acid ester, in a thin-film reactor.
Epoxy plasticizers, such as epoxidized unsaturated oils and, epoxidized alkyl fatty acid esters, particularly epoxidized soybean oil and epoxidized octyl esters of tall oil fatty acids can be manufactured by oxidation of either olefinic or aromatic double bonds, as follows: 
Hydrogen peroxide and the unsaturated oil or the alkyl fatty acid-ester alone do not react to any significant extent in prior art processes, and an organic peracid (usually acetic acid or formic acid) is necessary to shuttle the active oxygen from the aqueous phase to the oil phase. Once in the oil phase, the peracid adds oxygen across the carbon-to-carbon double bond and regenerates the original acid. On a commercial scale, epoxidation of soybean oil is achieved by oxidation of soybean oil with peracetic acid 
where the peracetic acid is derived from the oxidation of acetic acid with hydrogen peroxide, in-situ, in the presence of the soybean oil. In one process, peracetic acid generated in a process for oxidation of acetaldehyde to acetic acid has been isolated and used in the epoxidation process. This preformed peracetic acid can be handled, with proper precautions, in an inert solvent such as ethyl acetate or acetone. Others have found that an intermediate in the acetaldehyde oxidation process, acetaldehyde monoperacetate, also can be used as an epoxidizing agent. While oxidation of olefins by hydroperoxides is described in the literature, these prior art processes are far less efficient than the peracid processes.
The epoxidation processes can be divided into two basic types. In the first, the peracid is preformed; in the second, the peracid is formed in-situ in the primary reaction vessel. Representative schematics for the preformed and in-situ processes are shown in FIGS. 1A and 1B, respectively. The processes claimed herein are directed to a new and improved thin-film reaction process capable of efficiently reacting hydrogen peroxide directly with an unsaturated vegetable oil or alkyl fatty acid ester without first forming a peracid reactant.
The use of preformed peracetic acid results in epoxidation without catalyst at temperatures of 20 to 60xc2x0 C. at atmospheric pressure, as follows: 
The peroxidation of acetic acid with hydrogen peroxide is not efficient except at high molar ratios of acetic acid to hydrogen peroxide, resulting in large amounts of acetic acid to be recovered. In addition, concentrations of peracetic acid above 40 to 45 wt. % in acetic acid are explosive at epoxidation temperatures. Such processes require large volume production on an essentially continuous basis since the preformed peracid cannot be safely stored.
Experience has shown that the in-situ process is safer than processes using preformed peracids. In general, a peroxide solution (35% to 70% H2O2 in water) containing small quantities of a strong mineral acid catalyst, such as sulfuric acid or phosphoric acid, or styrene sulfonic acids, is added to a mixture of an epoxidizable substrate and acetic acid or formic acid at atmospheric pressure. As the reactants mix, the hydrogen peroxide and the acetic or formic acid react in the presence of the mineral acid catalyst to form the peracid, as follows: 
To prevent uncontrolled exotherm and to optimize epoxidation, the peroxide solution is added in several increments with agitation, and the reaction temperature is maintained at 50xc2x0 C. to 65xc2x0 C. for periods of 10 to 40 minutes per incremental addition of peroxide. One of the biggest problems with this process is that only small batch quantities of peracid can be formed in the presence of the unsaturated substrate. The peracid reacts with the unsaturated portion of the molecule and is quickly depleted, preventing a build-up of detonatable quantities of peroxide compounds, as follows: 
Further, significant problems are encountered in separation of the epoxidized substrate from the water, acid and peroxide remaining with the product after reaction. When the iodine number of the substrate has been reduced to the desired level, e.g., 0.5 to 10, the reaction is stopped and the epoxidized substrate is then difficult to separate from the aqueous layer since the aqueous layer contains a mixture of water, organic acid and some peroxide. Further, the epoxy layer contains acid catalyst that must be neutralized by a mild base, and residual peroxide that must be decomposed. The epoxy then is washed, centrifuged, decanted, filtered and transferred to a stripper where water and non-product residues are removed.
Most, if not all of the above-identified difficulties of the known epoxidation processes are eliminated, or substantially reduced in accordance with the processes described herein.
The processes described herein take advantage of thin-film reactor apparatus for epoxidizing an unsaturated substrate, particularly an unsaturated oil, such as soybean oil, linseed oil, or an alkyl ester of a fatty acid (hereinafter, the epoxidized unsaturated oil and/or the epoxidized alkyl fatty acid ester are referred to as the xe2x80x9cepoxidized substratexe2x80x9d) by directly reacting the unsaturated substrate with an oxidizing agent, such as hydrogen peroxide, acetaldehyde monoperacetate, an organic hydroperoxide, or a mixture thereof. The process can be operated continuously while continuously stripping most of the water from the epoxidized product in the thin-film reactor. Theoretically, the thin-film reactor processes described herein can strip sufficient water from the reaction product (epoxidized substrate) so that the epoxidized substrate requires little or no additional purification.
Accordingly, one aspect of the processes described herein is the rapid removal of water, in the vapor phase, to allow increased rates of reaction (epoxidation) of unsaturated compounds.
Another aspect of the processes is the elimination or substantial reduction of the neutralization, centrifuging, washing, decanting, and/or filtration steps needed with the presently practiced epoxidation processes.